Thermo-reversible gels and photographic emulsions prepared therewith



United States Patent THERMO-REVERSIBLE GELS AND PHOTO- GRAPHIC EMULSIONSPREPARED THERE- WITH Alain Charles Andr Clavier and Jacques Pouradier,

Vincennes, France, assignors to Eastman Kodak Contpany, Rochester, N.Y.,a corporation of New Jersey No Drawing. Filed Oct. 19, 1955, Ser. No.541,547

Claims priority, application France Dec. 8, 1954 3 Claims. (Cl. 96- 114)This invention relates to new products useful in the photographic artand a method of preparing them.

Gelatin has ordinarily been employed as the protective colloid for thelight-sensitive silver halides in photographic emulsions or ascomponents of other photographic layers (filter, antihalation,overcoating etc., layers) due to its ability to hold these salts insuspension, its good water permeability, its insolubility inphotographic developing solutions at ordinary temperatures and itsability to form a thin coating of the light-sensitive silver halideemulsion or other component upon a film support. Other protectivecolloids have been suggested for use instead of gelatin, but in mostcases, these products are not so satis factory as gelatin as far as oneor several of the abovementioned properties are concerned.

The most common method of preparing silver-halide dispersions is byreacting a water-soluble silver salt, such as silver nitrate, and awater-soluble halide, such as potassium bromide, in an aqueous solutionof a peptizing agent, usually gelatin. There results from this operationa dispersion of the silver halide in an aqueous solution of the gelatinor other peptizing agent, which solution also contains salts resultingas a by-product of the reaction. In processes of making silver-halidedispersions and emulsions using the natural product, gelatin, theemulsion maker has in the past been limited in the scope of hisoperations by the physical properties of that material. For instance,concentrations of silver halide and of gelatin have had to be used whichare not necessarily most desirable in photographic products. Also, inreducing the proportion of soluble salts in the emulsion to a minimummany difficulties have been met with. It has been the practicepreviously, where gelatin has been used as the dispersing material, toremove the soluble materials therefrom by setting the gelatindispersion, noodling and then Washing the noodles by osmosis in coldwater having a hardening agent therein. Under such conditions, a minimumgelatin concentration of approximately 4% is necessary, and thislimitation has hampered the operations of the emulsion maker in hispreparation of emulsions of this type.

Besides, the use of gelatin in silver halide photographic emulsions andother photographic materials is not satisfactory from other points ofview. Thus, gelatin can be attacked by animal and plant organisms. Itmay be attacked by insects, especially in tropical climates or by moldsor bacteria, especially under very humid conditions and at temperaturesconducive to their growth. Moreover, because of its natural source,gelatin varies in its properties from one batch to another. Further, thepreparation of gelatin sufficiently pure for photographic use demandsmuch care.

One object of our invention is to provide new products useful in thephotographic art, containing a thermoreversible gel which may be a totalor partial substitute for gelatin but which is without theabove-mentioned undesirable properties of gelatin. Another object of ourinvention is to provide a method of preparing these new products. Afurther object of our invention is to provide silver halide photographicemulsions in which the silver halide is suspended in thethermo-reversible gel referred to. A still further object of ourinvention is to provide a procedure for preparing useful thermalreversible gel compositions. Other objects of the invention will appearherein.

The new products relating to our invention comprise a thermoreversiblegel of a high polymer having a major proportion of acid and nitrilegroups.

In one embodiment of the invention, the high polymer present in theproduct in the form of a thermoreversible gel contains recurring unitsof the following general formula:

wherein R is a hydrogen or halogen atom or an alkyl su'bstituent group,and recurring units of the following general formula:

(II) G 112- C R- wherein R is a hydrogen or halogen atom or an alkylsubsti-tuent group, the ratio of recurring units of Formula II torecurring units of Formula I being 1:1 to 6:1 and, preferably, between1.5:1 and 3:1.

More particularly, the high polymer may be, for instance, a copolymer ofacrylic acid with acrylonitrile containing about one acrylic acid unitfor one to two acrylonitrile units.

It has been found that the above-defined high polymers formthermoreversible gels which have the property of precipitating in areversible manner for pH. values under 5 and to melt at temperatureswhich can be altered as desired within a broad range by merelycontrolling the conditions of preparation within the limits specifiedhereinafter. Besides, the properties of the gels are similar to those ofgelatin, as far as rigidity, transparency, swelling of the dried gel incold water and hardening properties are concerned.

The above properties, and other properties as indicated 7 hereinafter,make these gels particularly apt to be substituted for gelatin, eithertotally or partly, especially for photographic applications. Thecopolyrners prepared from the same monomers under conventionalconditions,

will generally not produce gels, and similar products in p the form of asolution or in unmixed form can be used only when mixed with gelatin. Itshould be noted, however, that the new gels of our invention, arecompatible with gelatin in any proportions. A method of preparing usefulthermoreversible gels is described herein.

The layer containing the macropolymers in the form of a gel can be anyof the layers which constitute a photographic product such as filter,anti-halation, overcoating, intermediate etc., layers. Such layers aretransparent and they can be hardened or made insoluble by means ofvarious reagents, such as salts of divalent metals, e.g. calcium.

The layer can also be a light sensitive silver halide emulsion. In sucha layer, the gel of macropolymer is used, for instance, as the binder orcarrier for the silver halides. Such a gel is particularly advantageoussince the layer of which it constitutes the binder is permeable to thephotographic processing baths generally used for processinggelatino-silver halide emulsions, and still re mains strong enough toresist the mechanical action of these baths.

The gel of macropolymer can be present in the emulsion layer, either asa peptizing agent for the silver halides, or as a binder for the silverhalides. It can be present both as a peptizing agent for silver halidesand as a binder, or only as a peptizing agent for the silver halides,the binder being hydrophilic colloid, such as gelatin, or as a binderfor silver halides peptized by conventional dispersing agents, such asgelatin, various gelatin derivatives such as those mentioned in the US.Patent 2,525,753 of August 13, 1947, hydrolyzed starch acetate,ethanolamine cellulose acetate, copolymers of maleic anhydride and vinylacetate, alkyl celluloses, and the like.

The support of the product of our invention may be of any usual basesuch as paper or conventional photographic film bases made from acellulose ester or a synthetic resin. On the nature of the supportdepends whethor an underlayer is necessary or useless between thesupport and the layer. For instance, an emulsion layer can be adjacentto the support provided the latter has no unfavorable eifect on the'emulsion. The emulsions may contain chemical and/ or optical sensitizersas usual emulsions do.

Our invention is not limited to the above-mentioned products but isgenerally applicable to all photographic products usually comprising alayer to which it was necessary, in the prior art, to incorporate forsome purpose gelatin or a substitute therefor. In products prepared inaccordance with our invention gelatin is partly or totally replaced by agel of a macropolymer of the type specified above. Advantageously, thisgel is substituted for all or the major part (at least 50%) of thegelatin, thanks to the thermoreversible properties of such a gel. Insome instances, it may be advantageous to add to the gel a smallquantity of gelatin as an auxiliary agent in view, for instance, of theaction of the gelatin on the photographic properties proper of theproduct in the case of a light sensitive layer.

Among other examples illustrative of the new products prepared inaccordance with our invention, the bichromated light-sensitive layers,receiving layers for dye imbibition printing, receiving layers used inreversal-transfer processes, and the like may be mentioned.

The preparation of new products in accordance with our invention isillustrated in Examples III to VII which should not be considered asexhaustive of our invention. The gels used in these examples aredescribed in Exampies I and II.

EXAMPLE I Two liters of water, 96 cc. of concentrated acrylic acid, 174cc. of acrylonitrile and, as the catalyst, a solution of 2 g. ofpotassium persulfate and 2 g. of sodium metabisulfite in 100 cc. ofwater were successively put into a vessel equipped with a mechanicalstirrer and maintained at 60 C. Stirring was continued at the sametemperature for 4 hours, and a few drops of hydrochloric acid were thenadded to enable the macropolymer to settle to the bottom of the vessel,in the form of a compact white paste. The macropolyrner was separatedfrom the liquid and thoroughly washed with water.

The resulting copolymer was dissolved in water, and a solution ofcaustic soda was added as dissolution was proceeding in such a way thatthe pH of the solution does not rise above and is comprised between 6and 10. The dissolution is faster at about 60 C. The proportion of wateradded was about ten times that of the macropolymer. Upon cooling, therewas obtained a transparent gel having a melting point of about 60 C.

EXAMPLE II A three-liter two-necked flask was placed on a water bath at65 C. One neck Was fitted with a dropping funnel. A thermometer wasintroduced through the other neck. In-the center was placed a stirrersurrounded by a condenser. Two liters of water were introduced in theflask, and when the temperature reached 65 C., 80 cc. of acrylic acidnitrile, 54 cc. of acrylic acid and'a solution of 1.6 g. of potassiumpersulfate and 1.6 g. of sodium metabisulfite in 100 cc. of water, weresuccessively introduced into the flask. Polymerization was virtuallycompleted within one hour.

The polymer suspension was set by adding a few drops of hydrochloricacid. The liquid was removed and the polymer was dissolved in an aqueousammonia-solution, or in water containing a normal solution of causticsoda so that the pH of the solution should not exceed 10 or 11. The purepolymer, having no free radicals and photographically inert, wasobtained upon a second acid precipitation. This polymer contains365/1000 (by weight) of acid groups and 635/1000 of nitrile groups. Itis soluble at pH values above 4.5 and insoluble at lower values of pH.The solution is extremely clear. The temperature of gelification is 27C. for a concentration of 6% and increases by 6 C. increments when theconcentration increases by increments. The gel is markedly moreresilient, but less rigid than gelatin. In the liquid form its viscosityis slightly greater than that of gelatin sol.

EXAMPLE III On the back side of a film having an emulsion layer on theother side, there was coated a sol of a polymer prepared under theconditions of Example I and corresponding to a gel whose melting pointis about 50 C. for an 8% concentration. Owing to the high meltingtemperature, the layer sets very rapidly which enabled coating to becarried out at a faster speed than the usual for coating of a gelatinbacking layer.

EXAMPLE IV To a melted gel prepared as described in Example I, therewere simultaneously added with rapid stirring in darkness, an aqueoussolution of silver nitrate and an aqueous solution of one or morealkaline halides (bromide, chloride and/or iodide); a few drops of anacid (sulfuric acid for instance) were added in order to reduce the pHto 24 whereupon the emulsion precipitated. After washing the pH wasadjusted to about 67 in order to obtain an emulsion ready for coatingonto a photographic base.

EXAMPLE V A polymer was prepared under the conditions of Example II. 2g. of this polymer were dissolved in 300 cc. of water adjusted to a pHof 5.5 and the mixture was heated at 40 C. 20 g. of potassium bromidedissolved in 60 cc. of water were added, and the temperature wasmaintained at 40 C. A solution of 19.5 g. of silver nitrate dissolved in100 cc. of water was prepared in a separate vessel and maintained at 40C. In darkness the solution of silver nitrate was added to the solutionof potassium bromide and the whole mixture was maintained at 60 C. forfive minutes; then, 30 g. of dry polymer were added with stirring andthe dissolution was rapidly completed. Stirring was continued for aquarter of an hour at 60 C. After this time the mixture was cooled andset. The resulting emulsion was coated on a paper support. Photographictests showed that the emulsion presented no fog effect and its speed wasequivalent to that of a similar emulsion prepared with an inert gelatin.

EXAMPLE VI EXAMPLE VII The following composition was coated on a grainedzinc plate Copolymer g 7 Ammonium bichromate g 3.5 Water; cc 100 Thelayer was exposed for three minutes through a negative illuminated by amercury vapor lamp (Mazda MA 500). A relief image was obtained bydevelopment with hot water. The copolymer employed in this example wasobtained by polymerizing at 65 C. for two hours a mixture of 160 cc. ofacrylic acid nitrile and 106 cc. of glacial acrylic acid in 4000 cc. ofwater, in the presence of a mixture of 3.2 g. of potassium persulfateand 3.2 g. of sodium metabisulfite.

Generally the products of our invention can be prepared by any knownprocess useful in preparing corresponding products containing gelatinexcept that the above copolymers are substituted for gelatin. The newproducts of our invention as shown in the preceding examples do notcause any fog to appear. They have no unfavorable action on the speedand contrast of the photographic emulsions. They are useful not only foremulsion layers but also for the other photographic layers such asbacking layers as described in Example III, overlayers, intermediatelayers, anti-halation layers, etc.

The macropolymer useful in the form of a gel in the products of ourinvention can be prepared by various means as indicated in the followingexamples and the various properties of thermoreversible gels can beadjusted to obtain the most advantageous properties of the productdesired by varying within the limits indicated the nature of the initialmonomers, the relative proportions of the monomers, the type of thepolymerization, the polymerization medium, the dilution of the monomers,the nature and proportion of the catalyst, the polymerizationtemperature and the like.

As regards the nature of the initial monomers acrylic acid andacrylonitrile are preferably used in view of their availability butother monomer mixtures can be used such as methacrylic acid andacrylonitrile or methacrylonitrile, acrylic acid and methacrylonitrile,etc. A small proportion (such as up to of a third copolymerizableethylenic monomer can be added to the mixture of the basic monomers inorder to alter the properties of the gel. For instance, the addition ofa small proportion of acrylamide to the initial monomers improves theadherence to glass of the thermoreversible gels of our invention whichmay be similar to that of gelatin. Likewise, the addition of a smallproportion of vinyl acetate will result in gels having an increasedrigidity while the addition of a small proportion of styrene will modifythe solubility properties of the gels. Also, it may be desired to haverecurring units reacting under the conditions of use of the new productsof our invention introduced in the molecule of the macropolymer, forinstance, recurring units apt to couple with the oxidation product of aprimary aromatic amine to form a dye.

The proportions of the two initial monomers are such that the copolymerformed will substantially comprise one acid group for l to 6 nitrilegroups and preferably for 1.5 to 3 nitrile groups. Of course, theproportions of the monomers mixed for the polymerization vary accordingto the conditions under which the polymerization is carried out asillustrated in the following examples.

The macropolymers can be obtained by copolymerizations of variousconventional types or by conversion of the functional groups of ahomopolymer. Where copolymerizations were used the macropolymers areprepared in a homogeneous phase in a medium wherein the monomers aresoluble and the polymers insoluble. An acrylonitrile-acrylic acidcopolymer can be obtained by partial hydrolysis of an acrylonitrilepolymer.

As regards the polymerization medium, an aqueous solution having a pH ofabout 3 is preferred. Preferably, the pH is adjusted to that value bymere addition of acrylic acid. The dilution varies from 5 to parts ofwater by Weight for one part of the monomer (by weight). However, otherpolymerization media can also be used such as ethyl alcohol, forinstance. The catalyst is preferably a mixture of equal proportions ofpotassium persulfate and sodium metabisulfite but other usual catalystscan also be used such as benzoyl peroxide or a mixture of hydrogenperoxide and ferrous chloride. The quantity of the catalyst used isgenerally comprised between and 7 of the weight of the monomers.

The polymerization is generally carried out at a temperature comprisedbetween 20 and C.

The copolymers are generally obtained in an aqueous acid medium. All ofthem are insoluble in such a medium and at the end of the polymerizationa white compact paste is obtained. In the case of alkaline hydrolysis ofpolyacrylonitrile, however, the polymer is in dissolved form. In eachinstance the polymer is precipitated by adjusting the pH to a valuewhich is at most equal to S, for instance, by the addition of sulfuricacid or hydrochloric acid. The polymer can then be washed with an excessof water whereby the traces of present catalyst, the non-reactedmonomers and the low-molecular portions are removed. After the polymerhas been washed, it is dissolved in water to which a base has beenadded. When a strong base is used such as caustic soda or potash the lyeis added dropwise during the neutralization of the acid groups of thepolymer. If a mild base is used such as ammonia, an excess of the basecan be added at once. The polymer is then stirred. For concentrationsabove it is necessary to heat the polymer during dissolution; this doesnot have a detrimental effect owing to the good heat stability of thepolymers.

Gelification is obtained by cooling the solution obtained as above. Thetemperature of gelification depends on the composition of the copolymer,as well as on the method of preparing the same and its concentration.

The following examples which should not be considered as limitative ofthe scope of our invention illustrate methods of preparing gels usefulin the products of our invention.

EXAMPLE VIII One liter of water, cc. of glacial acrylic acid, 200 cc. ofacrylonitrile and a catalyst solution of 1 g. of potassium persulfateand 1 g. of sodium metabisulfite in 100 cc. of water were successivelyintroduced in a vessel equipped with a stirrer and maintained at atemperature of 50 C. The temperature was maintained and stirring wascontinued for four hours; then, a few drops of hydrochloric acid wereadded. The polymer, which settled at the bottom of the container in theform of a white compact paste, was separated and carefully washed withwater.

A quantity of water forty times as great as the quantity of thecopolymer by weight and 50 cc. of 10 N ammonia were added to thecopolymer thus obtained. The mixture was heated to boiling temperatureto ensure rapid dissolution and this temperature was maintained untilthe ammonia smell was no longer noticeable. Upon cooling there wasobtained a gel melting at about 50 C. The polymer was dried in the formof the ammonia salt.

EXAMPLE IX Thermoreversible gels were obtained from the copolymerslisted in Table I by acidifying these copolymers and washing anddissolving them in an alkaline solution as described in Examples I andVIII.

All the copolymers of the present example are acrylic acid-acrylonitrilecopolymers prepared by using a mixture of equal proportions of potassiumpersulfate and sodiummetabisulfite as the catalyst. Column I indicatesthe ratio of the weight of pure glacial acrylic acid to the weight ofacrylonitrile, column II the Weight of the catalyst, in hundredths ofthe total Weight of the monomers, column III the amount of water addedto the monomers in terms of cubic centimeters by gram of monomer, columnIV the polymerizing temperature, and column V the polymerizing time inhours, that is to say the time during which the polymerizing conditionsindicated were maintained.

Table I I II III IV V C. 0. as 4.3 as as 3 0. 91 3.3 s. 1 60 3 0. 74 3.38.1 3 0. 59 3.3 a. 1 e0 3 0. 45 as 8.1 60 3 0. 74 0.8 8.1 24 s 0.74 1.68.1 40 4 0. 74 0.7 8.1 70 3 0. 67 3.3 8.1 60 3 0. 66 1.6 3.1 50 2EXAMPLE X By proceeding as in Example II, but using a mixture of 0.4 g.of potassium persulfate and 0.4 g. of sodium metabisulfite as thecatalyst, all other conditions being similar, the temperature ofgelification of the resulting product was 32 C. for a concentration of3.5% and increased by C. for every increase of the concentration byEXAMPLE XI A macropolymer similar to that described in Example II wasobtained by proceeding as in Example I! except that acrylic nitrile wasnot added until after polymerizing acrylic acid for one hour.Simultaneously with acrylic acid nitrile 0.5 g. of potassium persulfateand 0.5 g. of sodium metabisulfate were added. Sixty cc. of acrylic acidnitrile were suflicient to produce a copolymer giving a gel whoseproperties were similar to those of the gel described in Example II.

EXAMPLE XII This example illustrates the partial hydrolysis of a polymerof acrylic acid nitrile. To one part of waterinsolublc polymerizedacrylic acid nitrile in the form of a fine powder were added one part ofcaustic soda and 100 parts of Water. The mixture: was refluxed for onehour. The acrylic acid nitrile dissolved. The mixture in the flask wasadjusted to a. pH of about 9 and filtered. The polymer was obtained inthe liquid part by the addition of hydrochloric acid to a pI-l 2.5.Depending on the greater or lesser degree of the hydrolysis the polymergives gels whose melting temperatures are more or less high and whichare all the more rigid as the initial acrylonitrile polymer had agreater molecular weight.

EXAMPLE XIII Thermoreversible gels were obtained from polymers preparedas described in Example II except that the catalyst indicated in ExampleII was replaced by a solution of l g. of benzoyl peroxide in 50 cc. ofethanol and/or 'water was replaced by an equal quantity of ethanol.

EXAMPLE XIV A thermoreversible gel was obtained from a polymer preparedas described in Example II except previous neutralizing of the acrylicacid, adjusting of the initial pH of water to 10 and the use of 1 cc. ofhydrogen peroxide and 2.5 g. of ferrous chloride as the catalyst.

EXAMPLE XV Following the operating method described in Example IIpolymerization Was carried out at 30 C. in 1000 cc. of Water bysuccessively adding 87 cc. of acrylic acid nitrile and 48 cc. of acrylicacid, then 0.4 g. of potassium persulfate and 0.4 g. of sodiummetabisulfite in 50 cc. of water. The resulting polymer was similar tothe polymer of Example II and gave a gel of lower rigidity wlnch meltsat 45 C. for a concentration of /100; the melting temperature increasesby 6 for every increase of the concentration by 4 a The viscosity of thesol is smaller than that of the sol obtained from the polymer preparedin Example H.

8 EXAMPLE XVI By proceeding as in Example II except that 5 g. ofacrylamide dissolved in 20 cc. of water were added to the monomers, agel was obtained having similar properties as that of Example II andhaving also glass adherence properties similar to those of gelatin.

EXAMPLE XVII By proceeding as in Example II a copolymer was preparedfrom 90 cc. of acrylic acid, 90 cc. of acrylic acid nitrile and 15 cc.of vinyl acetate. When polymerization was completed and the pH adjustedto 10, the polymer Was hydrolyzed by heating to boiling temperature forone hour and subsequently treated as described in the above examples toobtain a gel. The resulting gel has a greater rigidity then gelatin andmelts at 36 C. for a concentration.

EXAMPLE XVIII By proceeding as described in Example II a copolymer wasprepared from cc. of acrylic acid, cc. of acrylic acid nitrile and 10cc. of styrene. The resulting polymer swells and is soluble in benzeneand water-acetone mixture.

Depending on the conditions of the preparation thermoreversible gels canbe obtained having gelification temperatures which range between 5 C.and 80 C. for concentrations comprised between 0.5% and 20%. Comparedwith photographic gelatin having a gelification temperature between 20C. and 35 C. for concentrations between 2% and 8% certain gels usefulfor carrying out the invention have substantially the same gelificati-otemperature as gelatin for the same concentrations while other gelswhich may be used in accordance with our invention have much higher orlower gelification tem peratures for the same concentrations or are moreconcentrated or diluted for the same gelification temperature. ExampleXIX illustrates this possibility of obtaining gels having differentmelting points in the same concentration.

EXAMPLE XIX By proceeding as indicated in Example II gels were preparedfrom acrylic acid nitrile and acrylic acid in proportions indicated inTable II with the indicated proportions of catalyst (mixture of equalproportions of potassium persulfate and sodium metabisulfite). Thepolymerization was carried out in two liters of water at thetemperatures indicated.

Table II Acrylic Acrylic Polymeriza- Relerencc No. of the gels nitrile,Acid, Satalyst, g. tion Temcc. cc. perature,

93 41 l 6 65 80 5t 1 6 65 80 54 0 1 65 186 82 3 2 50 93 41 l 6 50 80 540 1 50 For a concentration of 7% and pH 6, gel No. 1 melts at 24 C.; No.2 at 35 C. and No. 3 at 39 C.

For a concentration of 2% and pH 6, gel No. 4 melts at 20 C., No. 5 at44 C. and No. 6 at 67 C.

Likewise, gels can be prepared which will melt at a predeterminedtemperature in different concentrations.

Depending on the conditions of the preparation, it is possible to obtaingels having different rigidities and more particularly it is possible toobtain gels having the same gelification temperature but differentrigidities from macropolymers of different compositions. The rigidity ofthe gels varies depending in particular upon the temperature at which itis measured, the composition, and the method of preparation of themacropolymer. For

instance, if the rigidity is measured at 15 C. below the melting point,it increases from 1 to 20 when to 10% (by total weight of the monomer)of subsequently hydrolyzed vinyl acetate is introduced into the polymerat the beginning of the polymerization. When the proportion of vinylacetate added reaches 10% of the total weight of the monomers, therigidity of the corresponding gel is twice that of gelatin measuredunder the same conditions.

Similarly the viscosity of the sols corresponding to the gels which maybe used in accordance with the invention depends upon the conditions ofthe preparation. Viscosities are measured at various concentrations, ata temperature 10 C. above the melting point of the gel, and it appearsthat these viscosities are slightly greater than that of gelatinmeasured under similar conditions:

If the viscosity of gelatin is referred to as having the relative valueof 1, the viscosities for different polymers and differentconcentrations vary between 1.5 and 6 in relative value.

It is known that at the isoelectric point of gelatin the physical andmechanical properties of the latter such as gelification, rigidity,viscosity, turbidity and the like are at a maximum or a minimum. On theother hand, the gels used in accordance with the invention show in theirunhardened form the valuable characteristic to have substantiallyconstant properties between pH and pH 9. Under pH 5 they undergoreversible precipitation, that is to say, they can recover theiroriginal state if the pH is readjusted to its previous value, anadvantageous property for certain photographic applications.

Among the other advantages of the gels used in accordance with theinvention mention must be made of the fact that the corresponding solsproduce practically no foam while gelatin solutions readily produce foamand air bubbles. Moreover, while an aqueous gelatin solution is verysusceptible to bacterial action, the products of the invention either inthe form of gels or sols under the same conditions are not attacked.Some of these products can be preserved for several months at roomtemperature in the form of gels or sols without undergoing hydrolysisnor any apparent deterioration. Adding the products of the invention toa gelatin sol or gel will delay the bacterial attack on gelatin andunder usual conditions substantially eliminates it when the weight ofthe product of the invention added to gelatin is equal to the weight ofgelatin. Similarly, the sols of the invention can be maintained forseveral hours at boiling temperature without showing considerabledeterioration.

All these properties made the gels of the invention suitable for use inall the photographic fields of use of gelatin. Moreover, While most ofthese gels are compatible in all proportions with gelatin, they can beused in various mixtures with the latter especially for masking ordelaying the undesirable characteristics of gelatin such assusceptibility to bacterial attack as indicated above.

Also the gels described are useful in applications of gelatin other thanin the photographic field. The invention is of course not limited to theembodiments described which are only illustrative.

We claim:

1. A photographic material comprising a dispersion of silver halide in athermoreversible water solution of a polymer essentially consisting ofthe following recurring units:

CH2C R- 000M and --CH2CR- M being a substituent selected from the groupconsisting of alkali metal and ammonium, the ratio of the second unit tothe first unit being in the range of 1:1 to 6:1 said polymer beingessentially the sole gel-forming ingredient and binder for the silverhalide.

3. A method of preparing a silver halide dispersion which comprisesreacting silver nitrate with an alkali metal halide in athermoreversible water solution of a polymer which essentially consistsof the following recurring units:

R and R being selected from the group consisting of hydrogen, halogenand lower alkyl and M being selected from the group consisting of alkalimetal and ammonium, the ratio of the recurring units of formula II tothose of formula I being within the range of 1:11 to 6:1 said polymerbeing essentially the sole gel-forming ingredient and binder for thesilver halide.

References Cited in the file of this patent UNITED STATES PATENTS1,981,102 Hagedorn et al. Nov. 20, 1934 2,006,002 Schneider June 25,1935 2,123,599 Fikentscher et a1 July 12, 1938 2,461,023 Barnes et al.Feb. 8, 1949 2,504,049 Richards Apr. 11, 1950 2,522,771 Barnes et al.Sept. 19, 1950 2,541,474 Lowe et al. Feb. 13, 1951 2,566,149 Strain Aug.28, 1951 2,632,704 Lowe et al. Mar. 24, 1953 2,684,281 Lincoln July 20,1954 2,768,080 Hellmann Oct. 23, 1956 2,772,166 Fowler Nov. 27, 1956

1. A PHOTOGRAPHIC MATERIAL COMPRISING A DISPERSION OF SILVER HALIDE IN ATHERMOREVERSIBLE WATER SOLUTION OF A POLYMER ESSENTIALLY CONSISTING OFTHE FOLLOWING RECURRING UNITS: